The invention relates to a reinforcing cage for an element made of armoured concrete and also covers concrete elements comprising such a cage.
The invention relates especially to the realisation of prefabricated elements of armoured concrete.
In the field of construction and civil engineering, construction elements made of moulded concrete in which a metal armouring is imbedded, have been used for a long time.
It is known that the principle of armoured concrete consists in combining the qualities of concrete and of metal armourings. Usually, as indicated on FIG. 1, which illustrates the simple case of a part with rectangular straight section, subject to a load P, it is considered that a part made of armoured concrete comprises, on either side of a neutral axis xxe2x80x2x, two parts subject respectively to compression loads and to tensile loads. The latter are absorbed by a layer of longitudinal bars T whose cross-section is determined in relation to the loads applied and, in the case of a part subject to deflection, in relation to the distance (h) between the facing f1 of the compressed part and the centre of gravity of the armourings in tension T.
Besides, it is necessary to provide as well, in the compressed section, a layer of longitudinal bars that are linked with the bars in tension by transverse armourings, called stirrups enabling, in particular, to sustain shearing loads.
Generally, the reinforcement assembly of a concrete element therefore has the shape of a cage composed of two layers T, C of longitudinal bars respectively active and passive, linked together by stirrups E.
Most often, bars of both armouring layers are superimposed in planes parallel to the direction of the loads and spaced apart from one another, whereas the cage is thus composed of several parallel sections linked together by perpendicular joining bars whereas each section comprises two bars or groups of bars connected by stirrups.
In order to maintain relatively low prices, the armouring bars are produced in very large quantities in specialised facilities that have been arranged for manufacturing round bars of different sections, whereas the said bars can be twisted or corrugated for better adherence.
Going by catalogue, the user has only a limited number of types of bars of different sections available and, in order to obtain the section determined by calculation, it is often necessary to combine two or three attached bars together.
Moreover, the official rules impose to leave a minimum encasing distance between an armouring bar and the corresponding facing of the part, to prevent the concrete from corroding and bursting. Consequently, the armourings must be placed accurately inside the moulded part, while taking into account the diameter of the stirrups surrounding the bars.
When concrete is moulded on site, the first operation consists in laying formworks, called sheetings, which determine both facings of the wall and between which the reinforcing cage is mounted. Then, concrete is cast and it is necessary to wait for the concrete to set and harden before removing the formworks and starting the next building step. In the case of a slab, the frame is placed on a formwork, then concrete is cast.
To simply the construction and to obtain, moreover, excellent surface quality, it has been suggested for a long time to use prefabricated elements, realised in advance in a workshop especially fitted to that effect. Such a technique is valid, particularly, when a large number of identical parts must be realised, for instance in the building industry.
In this view, so-called heavy prefabrication techniques have been developed in order, for example, to realise standardised facing and floor elements. Indeed, for the construction of buildings, high capacity cranes are available, to install large dimension elements.
However, heavy prefabrication techniques have also been developed in civil engineering since mobile lifting vehicles are now available and enable to handle parts weighing several tons on the site.
For example, the inventor has developed, since 1981, an original technique for constructing conduits buried under an embankment that may provide civil engineering works of certain consequence for road or rail traffic.
In this technique, described in particular in the European patent no 0.081.402, the conduit consists of juxtaposed rings comprising each, as a cross-section, two side elements forming abutment walls and an upper curved element forming a vault resting on the ends of the said abutment walls.
If each ring is given a limited length, for example three meters, the corresponding elements can be placed on their longer side on a road trailer and be transported possibly over a long distance between the prefabrication works and the building site. Indeed, elements can be standardised and it is then profitable to build a factory fitted with moulds that can be used to realise a vast number of civil engineering works up to great distances away from the factory.
In all cases, for the armourings to work under the conditions foreseen by calculations, the said armourings must be positioned accurately inside the moulded part.
When parts are prefabricated in great numbers, the armourings are prepared in advance in specialised factories and delivered to the prefabrication workshop. The reinforcing cage of a prefabricated element must therefore exhibit sufficient rigidity in order to be handled and positioned inside the formwork.
To ensure interconnection of the different sections of a reinforcing cage, the longitudinal bars are generally bound with wire-ties or welded integral with the stirrups.
The realisation of reinforcing cages is therefore a rather delicate operation that must be performed by specialised staff, which increases the global cost of a prefabricated element, with respect to the cost price of metal bars.
Besides the possibilities of realisation depend, obviously, on the weight of the parts and on the lifting possibilities.
The technique for building buried elements described in particular in the patent EP-0.081.402 enables to use parts that are particularly thin with respect to their span. For example, the capacities of mobile cranes that can be used currently on building yards enable to handle upper elements with spans in excess of 10 meters.
Such a heavy prefabrication technique enables to carry out rapidly and cheaply civil engineering works of some importance, but the realisation and transport cost of the prefabricated elements plays a significant role in the cost price of the civil engineering work.
The purpose of the invention, without putting into question the calculation of the armouring and their general layout, consists in realising a new type of reinforcing cage that enables, notably to reduce the cost of the prefabricated elements and to increase, for equal weight, the sizes of the parts that conventional lifting vehicles enable to handle.
The invention applies especially to the realisation of civil engineering works consisting of curved prefabricated elements described in the previous patent EP-0.081.402. However, it has appeared that the reinforcing cages thus provided also exhibited advantages for other types of elements and, even, for on-site moulded concrete parts.
The invention therefore relates generally to a reinforcing cage for an armoured concrete element with two facings spaced apart from one another on either side of a neutral axis, between which a reinforcing cage is imbedded, comprising at least two layers of longitudinal armourings, respectively active and passive, substantially parallel, respectively to both facings and connected together by a transverse armouring, whereas each armouring has, as a cross-section, a determined area in relation to the loads to sustain in operation.
According to the invention, each longitudinal armouring consists of a flat iron with rectangular section whose width and thickness are determined in order to provide the area necessary to provide the strength required, with two plane faces, respectively an external face turned toward the corresponding facing and an internal face turned toward the neutral axis, and the transverse linking armouring between two opposite longitudinal armourings, respectively active and passive, consists of at least one elongated metal element, welded alternately on the internal faces of both corresponding flat irons.
According to another particularly advantageous feature, the spacing between the flat irons forming the longitudinal armourings, respectively active and passive, is determined in relation to the loads applied and each facing is placed at a minimum encasing distance from the external face of the corresponding longitudinal flat iron, in order to provide the concrete element with the thickness just necessary to provide the strength required.
Such a reinforcing cage remains conventional in its design and comprises therefore at least two armouring sections centred on planes at right angle to the facings and connected by joining bars. According to the invention, each section comprises at least two longitudinal flat irons spaced from one another and connected together by joining bars consisting of flat bands that cut the said longitudinal flat irons transversely and that are welded to the internal faces of the latter.
In this preferred embodiment, the transverse armourings for maintaining the spacing comprise at least one undulated band, welded alternately on the internal faces of both corresponding longitudinal flat irons of two layers, respectively active and passive.
In another embodiment, the transverse armouring comprises a series of distinct elements, each composed of a portion of band with two bent ends welded respectively on the internal faces of both longitudinal flat irons.
The invention applies especially to the realisation of a reinforcing cage for an element made of armoured concrete with two substantially parallel curved faces. In such a case, the longitudinal flat irons are curved so that their external faces are parallel, respectively to the corresponding facings of the element.
The invention also covers an element made of armoured concrete fitted with such a reinforcing cage and comprising, in a known fashion, two regions, respectively active and passive, on either side of a neutral axis. Each longitudinal bar of the reinforcing cage is then composed of a flat iron with an external face parallel to the corresponding facing of the element and spaced apart from the said facing by a minimum encasing distance (b). Thus, the thickness (H) of the element corresponding to the distance between the facings, respectively active and passive, can be limited to the value:
H=h+b+e/2
whereas (e) is the thickness of the active longitudinal flat iron and (h) the lever arm between the centre of gravity of the said flat iron and the passive facing of the element.
The invention also covers a method for realising a moulded element made of armoured concrete in which, in order to realise the longitudinal armourings, flat bars with rectangular cross-section are used, whose width and thickness are determined in order to form the area necessary to provide the strength required, whereas the said flat bars are positioned as foreseen for the armourings, respectively active and passive, and connected together by a transverse armouring comprising at least one band-shaped element, welded alternately on the internal faces, turned toward each other, of the said flat bars. The reinforcing cage thus formed is then placed in a mould delineating both facings whose spacing is determined in order to provide the thickness just necessary to maintain a minimum encasing distance between the external face of each longitudinal armouring and the corresponding facing.
But the invention will be understood better with the following description of certain particular embodiments given for exemplification purposes and represented on the appended drawings.